Laboratory tests were made to compare infection rates, reproductive capacity, and longevity of Anopheles albimanus Wiedemann when infections of Nosema algerae Vavra and Undeen were induced in each of the different larval instars. Infection rates of 100% were obtained in tests with 1st and 2nd instar larvae, and 88 and 92%, respectively, were obtained in tests with 3rd and 4th instar larvae. Infections of N. algerae sharply reduced the reproductive capacity and longevity of An. albimanus regardless of the instar that became infected, but the reductions were greatest when 1st and 2nd instars were exposed. The tests indicate that the introduction of N. algerae into natural populations of An. albimanus consisting of mixed larval instars could significantly reduce the number and fertility of eggs, and the thus result in an average reduction of ca. 54% of the F1 progeny from these females that become infected.